A driver assistance system provides one or multiple accessory equipment items in a vehicle to support the driver in certain driving situations. Such additional equipment includes, for example, an ABS (antilock brake system), an ESP (electronic stability program), an ACC (adaptive cruise control) or a parking assistance system.
The use of a rain sensor for automatic activation of the windshield wipers of a vehicle when it is raining belongs to the area of driver assistance. In this regard, German Patent No. DE 103 16 794 A1 describes a rain sensor which includes a camera. The camera is directed from the interior at an object fixedly mounted on the vehicle in front of the windshield, for example, the hood of the vehicle. The image recorded by the camera is compared with a stored image recorded when the windshield was dry. The presence of moisture, e.g., in the form of droplets on the windshield, is detected by comparing the images.
However, the rain sensor described above has the disadvantage that it generally responds only to rain. If it is not raining (any longer), the rain sensor will consequently detect dryness. Such a sensor may be used to control the windshield wiper but it can hardly be used appropriately to control other components of the vehicle in which an adaptation to moisture in the surroundings of a vehicle would be meaningful. For example, automatic control in response to a wet road surface cannot be derived from the rain sensor data: the road surface might still be wet or might already be wet even if it is not raining at the moment.
German Patent No. DE 101 497 68 A1 describes a method for determining the visual range of a laser scanner mounted on a front side of a vehicle. A sensor receives radiation emitted by the scanner into a monitoring area in front of the vehicle and then reflected from there. An evaluation unit evaluates the reflected radiation. Interference such as rain fog or spray from a preceding vehicle is detected. Measures are then taken in response to this detection. These measures may include, for example, intervention into operation of the vehicle, for example, by reducing the instantaneous vehicle speed or a predefined maximum speed.
The scanner described above provides information about wetness which might be present on the road more reliably than does a rain sensor. However such a system is complex and is therefore expensive.